1. Field of the Invention
This invention relates to a process for placing electrically conductive paths on a substrate. The process of this invention is particularly useful for the manufacture of printed circuit boards.
2. Prior Art
Electrical instrumentation utilizes circuit boards for connecting together the components of its circuitry. Each circuit board is comprised of a dielectric substrate having on the surface a plurality of electrically conductive paths, arranged in a pre-determined manner, to connect together circuit elements mounted on the substrate. A number of procedures have been developed for manufacturing printed circuit boards. For example, in one method, the conductive paths may be placed on the substrate by the application of a metal conductor to the substrate and the subsequent removal of portions of the conductor by liquid chemicals or by depositing an electrical conductor on the substrate with chemical plating baths. Examples of other methods of manufacturing circuit boards may be found in U.S. Pat. Nos. 4,327,124 and 3,013,913. In these methods which involve the use of chemicals a number of problems exist. For example, chemical wastes are a problem as the wastes must be disposed of without adversely affecting the environment. Moreover, chemicals for use in these methods are relatively expensive, and these methods are complex such that only a few companies are capable of producing printed circuit boards using these methods.
Various attempts have been made to manufacture printed circuit boards through use of powdered metallurgical techniques. For example, U.S. Pat. No. 2,993,815 teaches the application of a copper and glass containing paste to a refractory substrate in the desired pattern, followed by a heat treatment to sinter the metal and cause it to bond to the substrate. Another powdered metallurgical procedure is described in U.S. Pat. No. 3,256,109 in which a metallic compound is initially atomized with the material making up the substrate, with the circuit pattern formed by thermal decomposition induced by a heated die. Yet another powdered metallurgical approach is disclosed in U.S. Pat. No. 3,410,714, in which a circuit forming material, such as powdered copper, is applied to a substrate in the desired pattern, whereupon the structure is heated at a relatively high temperature to cause alloying and bonding of the metal to the substrate. Still another powdered metallurgical method of manufacturing printed circuit boards is disclosed in U.S. Pat. No. 3,800,020. In this procedure, a printed circuit board is formed by applying a thin layer of powdered metal composed of a mixture of copper-tin-lead to a heat softenable substrate, followed by application of a heated die having the desired circuit pattern thereon to sinter the metal particles in the areas contacted with the circuit defining portions of the die and to soften the substrate to effect a bond between the sintered metal circuit pattern and the substrate. Each of these powdered metallurgical methods suffer from a number of inherent disadvantages. For. example, in some of these procedures, the resulting metal printed circuit contains dispersed materials from the substrate, resulting in a circuit that does not have the conductivity necessary for certain circuit board applications. Other of these methods are complicated and expensive, and do not readily lend themselves to mass production techniques.